1. Field of the Invention
Aspects of the present invention relate to a holographic storage medium and, more particularly, to an apparatus and method of reducing interpage reproduction crosstalk noise in a holographic storage medium.
2. Related Art
In optical holography, data is stored in the volume of a recording medium instead of on the surface of the recording medium. A beam incorporating a data signal interferes with a reference beam, producing interference gratings (called data pages) inside the recording medium. The gratings overlap each other by changing optical characteristics of the reference beam through a process called multiplexing. To read data from the recording medium, a single reference beam illuminates the recording medium under the same conditions as those used for data recording, generating a diffraction beam representing a stored data page. The diffraction beam is detected by a detection array, which extracts a stored data bit from a measured intensity pattern. Each of the stored data pages contains a plurality of data bits or pixels. Overlapping the data pages in the same volume may increase the recording medium's storage capacity.
A hologram is recorded using both a signal beam and a reference beam. As shown in FIG. 1, during recording, a reference beam R and a signal beam S interfere with each other to generate an interference pattern. The interference pattern is transmitted to a medium. During reproduction, the original reference beam R that was applied for recording illuminates the hologram recorded in the medium, causing diffraction in the recorded hologram and producing an output signal beam S. Hence, if the reference beam for reproduction differs from the reference beam for recording, the reproduced beam also differs from an initially recorded beam in intensity and direction. Conventionally, as the difference in intensity and direction between the reproduced beam and the initially recorded beam increases, the intensity decreases in the form of a sinc function.
FIGS. 2A and 2B are views illustrating a grating recorded by a pair of plane waves, and an angular change of a reproduced beam and a decrease in diffraction efficiency that occurs since a reference beam for reproduction does not satisfy a certain condition. In FIGS. 2A and 2B, {right arrow over (K)}S and {right arrow over (K)}R are wave vectors of a signal beam and a reference beam, respectively, and {right arrow over (K)} is the vector of the grating formed by interference between the signal and reference beams. L denotes the thickness of a medium and {circumflex over (z)} denotes a unit vector in a direction perpendicular to the medium. In FIG. 2B, a dotted line denotes a recording beam and a solid line denotes a reproduction beam. Conventionally, as shown in the equation of FIG. 2B, the diffraction efficiency (η) decreases, as each of the wave vectors of the signal beam and the reference beam does not satisfy Bragg conditions. The Bragg conditions indicate that high diffraction efficiency is provided when holograms recorded with a specific reference beam are reproduced with the same reference beam. When the wave vector of the reference beam used for reading differs substantially from that of the reference beam for recording, the diffraction efficiency decreases significantly. The wave vector of the reference beam for reading may be changed by changing an incident angle or wavelength of the reference beam.
Due to this phenomenon, a holographic storage medium records signals of several overlapping data pages in the same volume of the medium. When recording overlapping signals, the reference beam is different for each of the overlapping data pages. Overlapping data pages are reproduced with varying intensity depending on the reproducing reference beams incident on the holographic storage medium. Therefore, the reproduced data pages can be distinguished from each other by modulating the condition of the reference beam. However, in actual application, it is difficult to separate the data pages from each other perfectly using the different reference beams used to record the respective data pages. Particularly, in order to increase recording capacity, the interval of the reference beams used for recording is small. For this reason, in actual reproduction, both a first data page and a second data page recorded under close conditions are reproduced, thereby causing crosstalk (noise) and reduced signal quality.
To reduce crosstalk, the conventional art has only addressed the issue of crosstalk from an optical point of view in terms of how the difference between reference beams can be maximized. Thus, a method of maintaining the maximum difference between reference beams corresponding to respective data pages has been employed.
That is, an optical method has been used to simply widen an interval between data pages when multiplexing recording and reduce crosstalk only in units of a page group consisting of a plurality of recorded data pages. However, when sufficient intervals between the plurality of recorded pages of the page are not ensured, signal quality greatly deteriorates due to crosstalk. For example, in the conventional optical methods widening the intervals of the data pages has been considered to be sufficient because the recording capacity is not large. However, a method of removing crosstalk more efficiently is being demanded in order to increase the recording capacity in the future.